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# RF Power Amplifier Design

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� Basic Amplifier Concepts l Class A, B, C, F, hHCA l Linearity Aspects l Amplifier Example � Enhanced Amplifier Concepts l Feedback, Feedforward, ... l Predistortion l LINC, Doherty, EER, ...

• pg 1
```									RF Power Amplifier Design

Markus Mayer & Holger Arthaber
Department of Electrical Measurements and Circuit Design
Vienna University of Technology

June 11, 2001

Contents

¤ Basic Amplifier Concepts
l Class A, B, C, F, hHCA
l Linearity Aspects
l Amplifier Example

¤ Enhanced Amplifier Concepts
l Feedback, Feedforward, ...
l Predistortion
l LINC, Doherty, EER, ...

2
Efficiency Definitions

POUT
¤ Drain Efficiency:                 ηD =
PDC

¤ Power Added Efficiency: η PA = POUT − PIN = η D ⋅ 1 − 1 
      
PDC              G

3

Ideal FET Input and Output Characteristics

IDS
VGS=0
Im

gm

VGS=VP
VGS                                    VDS
2VP   VP        0          0   VK      VDD         VDSmax

Ohmic      Saturation   Breakdown

VDD − V K
κ=
VDD

4
Maximum Output Power Match
I DS
VGS=0
Im

gm

VGS=VP
VGS                                        VDS
2VP   VP        0          0   VK      VDD             VDSmax

Ohmic       Saturation      Breakdown

VDS max − VK
ROPT =
Im

5

Class A
IDS                                          IDS

Im                                             Im

VGS                                        VDS                  Q
2VP   VP        0          0   VK           VDD     VDSmax              0   p   2p
VGS                                        VDS

p

2p
Q

6
Class A – Circuit
VDD

G    D
RL
S

η D = κ ⋅ 50%

G = G A (e.g. 14 dB)

η PA = κ ⋅ 48%

7

Class B
IDS                                        IDS

Im                                          Im

VGS                                      VDS                 Q
2VP   VP   0          0   VK        VDD         VDSmax          0   p   2p
VGS                                      VDS

p

2p
Q

8
Class C
IDS                                      IDS

Im                                       Im

VGS                                   VDS                  Q
2VP   VP   0          0   VK          VDD   VDSmax            0   p   2p
VGS                                   VDS

p

2p
Q

9

Class B and C – Circuit
VDD

f0

G    D
RL
S

Class B                         Class C
ηD = κ ⋅ 78%                    ηD → 100%

G = G A - 6dB (8 dB)            G →1

ηPA = κ ⋅ 65%                   ηPA → 0%
10
Influence of Conduction Angle

11

Class F (HCA ... harmonic controlled amplifier)
IDS                             IDS

Im                               Im

VGS                           VDS                 Q
2VP   VP   0          0   VK   VDD   VDSmax          0   p   2p
VGS                           VDS

p

2p
Q

12
hHCA       (half sinusoidally driven HCA)

IDS                                            IDS

Im                                              Im

VGS                                          VDS                  Q
2VP   VP   0          0    VK    VDD           VDSmax               0    p   2p
VGS                                          VDS

p

2p
Q

13

Class F and hHCA – Circuit
VDD

Zo(n)

0, n=1
ID    Ze(n)         inf, n=odd

VDS          0, n=even
RL
inf, n=even

Class F                                 hHCA
ηD = κ ⋅100%                         ηD = κ ⋅100%

G = G A - 5dB (9 dB)                 G = G A + 1dB (15 dB)

ηPA = κ ⋅ 87%                        ηPA = κ ⋅ 96%
14
hHCA – Third Harmonic Peaking
IDS                                          IDS

Im                                            Im

VGS                                        VDS                 Q
2VP   VP   0          0    VK         VDD      VDSmax             0   p   2p
VGS                                        VDS

p

2p
Q

15

Third Harmonic Peaking – Circuit
VDD

G    D             3f0
f0            RL
S

ηD = κ ⋅ 91%

G = G A + 0.6dB (14.6 dB)

ηPA = κ ⋅ 87%

16
Linearity Aspects

17

Linearity Aspects

¤ Class A         ¤ Class AB

¤ Class B         ¤ Class C

18
Linearity Aspects

¤ Ideal strongly nonlinear model   ¤ Strong-weak nonlinear model

19

Amplifier Design – An Example
¤ Balanced Amplifier Configuration

Port 1
Z=50 Ohm                                           Port 2
Z=50 Ohm

20
Amplifier Design – Simulation
¤ Gate & Drain Waveforms

Gate waveforms                                                                                         Drain waveforms
1                                                                                                      1000     25                                    Inner Drain Voltage (L, V)   Inner Drain Current (R, mA)
5000
Amp                          Amp

20                                                                                                  4000
0                                                                                                      500
15                                                                                                  3000

-1                                                                                                     0        10                                                                                                  2000

5                                                                                                  1000
-2                                                                                                     -500
Inner Gate Voltage (L, V)         Inner Gate Current (R, mA)
0                                                                                                  0
Amp                               Amp

-3                                                                                                     -1000    -5                                                                                                  -1000
0                                   500                                1000                1300                 0                       500                                   1000                          1300
Time (ps)                                                                                           Time (ps)

21

Amplifier Design – Simulation
¤ Dynamic Load Line & Power Sweep

Dynamic load line                                                                                 Power Sweep 1 Tone
8000                                                                                                            40                                                                                                       80
IVCurve (mA)
Output Power (L, dBm)
IV_Curve                                                                                                                                   70
Amp
6000                                                                          Dynamic Load Line (mA)                     PAE (R)
Amp                               30       Amp                                                                                             60

4000                                                                                                                                                                                                                     50

20                                                                                                       40
2000                                                                                                                                                                                                                     30

10                                                                                                       20
0
10

-2000                                                                                                            0                                                                                                       0
0                    3                   6                     9             12                   15        0          5                  10           15                                  20              24
Voltage (V)                                                                                   Power (dBm)

22
Amplifier Design – Measurements
¤ Single Tone & Two Tone

60                                                  60 PAE[%]
40                                                            A %
80 P E[ ]

1dBCP
35                                                        70
50                                                  50

30                                                        60

P out [dBm], IMDD [dBc], Gain [dB]
40                                                  40
P out [dBm], Gain [dB]

25                                                        50

Pout                                                                                                      Pout
IMDD
Gain                                           30                                                  30
20                                                        40                                                                                                              Gain
GammaIn
PAE
A
PE
15                                                        30
20                                                  20

10                                                        20

10                                                  10
5                                                    10

0                                                     0                                                   0                                                    0
0   5   10   15              20   25       30   35                                                         0   5   10   15                20   25   30   35
Pin[dBm]                                                                                                   P in [dBm]

23

Amplifier Nonlinearity
¤ Gain and Phase depends on Input Signal

¤ 3rd Order Gain-Nonlinearities:

24
Amplifier Nonlinearity
¤ Higher Output Level (close to Saturation) results
in more Distortion/Nonlinearity

25

¤ Generation of Harmonics

¤ Intermodulation Distortion / Spectral Regrowth

¤ Constellation Deformation

26
Intermodulation and Harmonics

27

Spectral Regrowth
10
ACPR 1>60dB
ACPR 2>60dB
0
ACPR 1=16dB
ACPR 2=43dB
-10
relative power / dB

-20

-30

-40

-50

-60
-15   -10   -5            0         5          10     15
relative frequency / MHz

¤ ACPR (Adjacent Channel Leakage Power Ratio) increases

28
Reduced NPR             (Noise Power Ratio)

¤ Input Signal                ¤ Output Signal of
Nonlinear Amplifier

¤ „Noisy“ Constellation

29

Constellation Deformation
¤ Input Signal                ¤ Output Signal of
Nonlinear Amplifier
(with Gain- and Phase-Distortion)

30
Modeling of Nonlinearities
¤ with Memory-Effects
l Volterra Series (=„Taylor Series with Memory“)

¤ without Memory-Effects
αar                 α Θr 2

performance
l Saleh Model f (r ) =            g (r ) =
1 + β ar 2          1 + βΘr 2

better
l Taylor Series
l Blum and Jeruchim Model
l AM/AM- and AM/PM-conversion

31

AM/AM- and AM/PM-Conversion
¤ GaAs-PA

32
AM/AM- and AM/PM-Conversion
¤ LDMOS-PA

33

How to preserve Linearity?
¤ Backed-Off Operation of PA
l Simplest Way to achieve Linearity

¤ Linearity improving Concepts
l Predistortion
l Feedforward
l ...

34
How to preserve Efficiency?
¤ Efficiency improving Concepts
l Doherty
l Envelope Elimination and Restoration
l ...

¤ Linearity improving Concepts
l Higher Linearity at constant Efficiency
à Higher Efficiency at constant Linearity

35

Direct (RF) Feedback

¤ Classical Method
¤ Decrease of Gain à Low Efficiency
¤ Feedback needs more Bandwidth than Signal
¤ Stability Problems at high Bandwidths
36
Distortion Feedback

¤ Feedback of outband Products only
¤ Higher Gain than RF feedback
¤ Stability Problems due to Reverse Loop

37

Feedforward

¤ Overcomes Stability Problem by forward-only Loops
¤ Critical to Gain/Phase-Imbalances
0.5dB Gain Error à -31dB Cancellation
2.5° Phase Error à -27dB Cancellation
¤ Well suited for narrowband application

38
Cartesian Feedback
baseband input
I
modulator     main amp.
I                                                         RF-output
OPAs
Q
Q

local
oscillator
10
UMTS example:
original signal
I                                                                                            predistorted signal
Q                                                      0

demodulator                                          -10

relative power / dB
-20

¤ AM/AM- and                                                                -30

AM/PM-correction                                                          -40

¤ High Feedback-Bandwidth                                                   -50

¤ Stability Problems                                                        -60
-30      -20   -10          0        10       20           30
relative frequency / MHz
39

Digital Predistortion
¤ Digital Implementation of „Cartesian Feedback“
¤ Loop can be opened à no Stability Problems

40
Analog Predistortion

¤ Predistorter has inverse Function of Amplifier
¤ Leads to infinite Bandwidth (!)
¤ Hard to realize (accuracy)

41

Analog Predistortion
¤ Possible Realizations:

42
LINC      (Linear Amplification by Nonlinear Components)

s1(t)       Ks1(t)
K
K(s1(t)+s2(t))
s(t)       signal                                                    =Ks(t)
separation

s2(t)       Ks2(t)
K

UMTS example:
10
¤ AM/AM- and                                                               s(t)
ACPR 1 >60dB
ACPR 2 >60dB

AM/PM-correction                                                 0       s1 (t)
ACPR 1 =18dB
ACPR 2 =29dB

¤ Digital separation required
-10

relative power / dB
(accuracy!)                                                    -20

¤ High Bandwidth,                                                -30

oversampling necessary                                         -40

¤ Stability guaranteed                                           -50

-60
-30   -20       -10          0        10     20       30
relative frequency / MHz
43

Doherty Amplifier
¤ Auxiliary amplifier supports main amplifier during saturation
¤ PAE can be kept high over a 6dB range

44
Doherty Amplifier
¤ Gain vs. Input Power                                                       ¤ Efficiency vs. Input Power
POUT

2)
+A
(A1
n
io       main amp. (A1)
at
ur
ig
nf
co
rty                         aux. amp. (A2)
he
do

PIN

¤ No improvement of AM/AM- and AM/PM-distortion
¤ Behavior of auxiliary amplifier very hard (impossible) to realize
¤ Stability guaranteed

45

EER                       (Envelope Elimination and Restoration)

¤ Separating phase and magnitude information
¤ Elimination of AM/AM-distortion
¤ Application of high-efficient amplifiers
(independent of amplitude distortion)
¤ Stability guaranteed
amplitude information

RF input           signal
separation
phase information                RF output

high efficiency
power amplifier

46
EER                          (Envelope Elimination and Restoration)

¤ Analog realization                                                                                               peak detector
supply voltage
amplifier

l Limiter hard to build
l Accuracy problems                                                                                                                                  limiter
l Feedback necessary                                                       RF input                                                                                                               RF output

high efficiency
peak detector       power amplifier

¤ Digital realization
l Oversampling + high D/A-                                                                                                                  amplitude information
conversion rates required                                                                                                                D
supply voltage amplifier

digital baseband input
l High power consumption                                                                        I
A

of DSP and D/A-converters                                                                               digital                          D                    modulator
signal
l Possible feedback                                                                             Q        processor                               A             I                                 RF output
Q
elimination                                                                                                                              D
high efficiency
l Compensation of AM/PM-                                                                                                                         A                              power amplifier
phase information
distortion possible                                                                                                                                                   local oscillator

47

EER                          (Envelope Elimination and Restoration)

¤ Bandwidth of Magnitude- and                                                                        ¤ Five times (!) oversampling
phase-signal have higher than                                                                        necessary to achieve standard
transmit signal                                                                                      requirements

UMTS example:                                                                                              UMTS example:
10                                                                                                         10
full bandwidth                         ACPR1 >60dB
Magnitude
Phase                                                                     3⋅B0 bandwidth                         ACPR2 >60dB
0                                                                                                          0
5⋅B0 bandwidth                         ACPR1 =33dB
7⋅B0 bandwidth                         ACPR2 =40dB
relative power / dB

-10                                                                                                        -10
relative power / dB

ACPR1 =51dB
ACPR2 =36dB
-20                                                                                                        -20
ACPR1 =53dB
ACPR2 =49dB
-30                                                                                                        -30

-40                                                                                                        -40

-50                                                                                                        -50

-60                                                                                                        -60
-30    -20   -10          0        10    20                                30                              -30    -20       -10          0        10          20        30
relative frequency / MHz                                                                                       relative frequency / MHz

48
¤ Varying/Switching of Bias-Voltage depending on
Input Power Level
¤ Selection of Operating Point with high PAE
¤ Applicably for nearly each type of Amplifier
peak detector
bias
control

RF input                                                   RF output

high efficiency
power amplifier

49

¤ Single tone PAE for switched                                            ¤ Simply to implement Concept
VDD with VG kept constant                                               ¤ Stability guaranteed
90                                                     ¤ Possible problems:
80                                                        l DC-DC converter with high

70                                                          efficiency necessary
60                                                        l Possible Linearity Change
50
(can increase and decrease)
40
especially for HCAs
V D =3.5V
30                                         V D =4.5V
V D =6.5V
20
32   33   34   35      36     37     38   39     40
output power / dBm

50
Summary
¤ Digital Realization required to achieve Accuracy

¤ Problem of Stability for high Bandwidth Application

¤ Higher Bandwidths (Oversampling) necessary,
depending on Order of IMD cancellation

¤ Predistortion gives best Results while keeping
Efficiency high (valid for high Output Levels > 40dBm)

51

Figure References
¤ F. Zavosh et al,
“Digital Predistortion Techniques for RF Power
Amplifiers with CDMA Applications”,
Microwave Journal, Oct. 1999

¤ Peter B. Kenington,
“High-Linearity RF Amplifier Design”,
Artech House, 2000

¤ Steve C. Cripps,
“RF Power Amplifiers for Wireless Communications”,
Artech House, 1999

52
Contact Information

DI Markus Mayer              DI Holger Arthaber

( +43-1-58801-35425            ( +43-1-58801-35420

- markus.mayer@tuwien.ac.at   - holger.arthaber@tuwien.ac.at

53

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